Integrated circuit connection arrangement for minimizing crosstalk

ABSTRACT

A semiconductor package includes a leadframe, having perimeter package leads and a ground voltage lead, a bottom semiconductor die flip-chip mounted to the leadframe, and a top semiconductor die. The bottom semiconductor die has a first frontside active layer with first frontside electrical contacts electrically connected to the leadframe, a first backside portion, and a buried oxide layer situated between the first frontside active layer and the first backside portion. The top semiconductor die is mounted to the first backside portion. The first frontside active layer includes a circuit electrically connected to the first backside portion by a backside electrical connection through the buried oxide layer. The first backside portion of the bottom semiconductor die is electrically connected to the ground voltage lead through a first electrical contact of the first frontside electrical contacts to minimize crosstalk.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.15/853,357, filed Dec. 22, 2017 and entitled “Integrated CircuitConnection Arrangement For Minimizing Crosstalk,” which claims thebenefit of U.S. Provisional Application No. 62/461,117, filed Feb. 20,2017, and entitled “Backside Contact Integrated Laterally Diffused MOSApparatus and Methods,” all of which are incorporated by referenceherein in their entirety.

BACKGROUND

Semiconductor power devices are specialized devices that are typicallyused as switches or rectifiers in power electronics circuits.Semiconductor power devices may be implemented using lateral diffusionfield effect transistors (LDFETs), such as lateral diffusion metal oxidesemiconductor (LDMOS) transistors. These types of transistors arecharacterized by a “lateral diffusion” region (or low-doped orlightly-doped drain (LDD) region) that corresponds to an extension ofthe drain region that is less strongly doped than the core drain region,and that extends laterally away from the channel. The lateral diffusionregion increases an LDFET's ability to handle higher voltages in theoff-state by absorbing portions of the electric field that wouldotherwise cause source-drain punch-through, and to handle largercurrents in the on-state by preventing a large potential drop frombuilding up at the drain-body interface which would otherwise result indegradation of the device via the injection of hot carriers into thebody of the device.

A switching voltage regulator typically includes two semiconductor powerdevices that constantly switch on and off in a synchronized manner toregulate a voltage. This switching may create electrical interferencethat negatively impacts surrounding circuitry, manifesting, in somecases, as crosstalk. Crosstalk is a phenomenon caused by undesirable(e.g., inductive, capacitive, or conductive) coupling from one circuitto another. In tightly-packed semiconductor packages where integratedcircuits for control of power electronics circuits are packaged alongwith the power electronic circuits themselves, crosstalk between thepower electronic circuits and the controller circuits may result inundesirable system performance or behaviors.

SUMMARY

In some embodiments, an integrated circuit (IC) package includes aleadframe having perimeter package leads and a ground voltage lead, abottom semiconductor die, and a top semiconductor die. The bottomsemiconductor die is flip-chip mounted to the leadframe. The bottomsemiconductor die includes: i) a first frontside active layer havingfirst frontside electrical contacts electrically connected to theleadframe, ii) a first backside portion, and iii) a buried oxide layer.The buried oxide layer is situated between the first frontside activelayer and the first backside portion. The top semiconductor dieincludes: i) a second frontside, and ii) a second backside. The secondbackside of the top semiconductor die is mounted to the first backsideportion of the bottom semiconductor die. The first frontside activelayer of the bottom semiconductor die includes a circuit electricallyconnected to the first backside portion by a backside electricalconnection through the buried oxide layer. A first electrical contact ofthe first frontside electrical contacts is electrically connected to thebackside electrical connection, and the first backside portion of thebottom semiconductor die is electrically connected to the ground voltagelead of the leadframe through the first electrical contact to minimizecrosstalk.

In some embodiments, a method for packaging a semiconductor device in anIC package involves providing a leadframe having perimeter package leadsand a ground voltage lead. A bottom semiconductor die is formed, thebottom semiconductor die including: i) a first frontside active layerhaving first frontside electrical contacts, ii) a first backsideportion, and iii) a buried oxide layer. The buried oxide layer issituated between the first frontside active layer and the first backsideportion. The bottom semiconductor die is flip-chip mounted to theleadframe. The flip-chip mounting comprises electrically connecting andphysically mounting the first frontside electrical contacts of thebottom semiconductor die to the leadframe, a first electrical contact ofthe first frontside electrical contacts being electrically connected andphysically mounted to the ground voltage lead of the leadframe. A topsemiconductor die is provided. The top semiconductor die includes: i) asecond frontside having second frontside electrical contacts, and ii) asecond backside. The top semiconductor die is mounted to the bottomsemiconductor die, the second backside of the top semiconductor diebeing attached to the first backside portion of the bottom semiconductordie. The first frontside active layer of the bottom semiconductor dieincludes a circuit electrically connected to the first frontsideelectrical contacts by respective first frontside electrical connectionsand electrically connected to the first backside portion by a firstbackside connection through the buried oxide layer. The first backsideportion of the bottom semiconductor die is electrically connected to theground voltage lead of the leadframe through the first backsideconnection and the first electrical contact of the first frontsideelectrical contacts to minimize crosstalk.

DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram of an example of a high-power semiconductorswitch incorporating some embodiments.

FIG. 2A is a simplified diagrammatic top orthographic view of anintegrated circuit package, in accordance with some embodiments.

FIG. 2B is a simplified diagrammatic cross-section of an integratedcircuit package, in accordance with some embodiments.

FIG. 2C is a simplified diagrammatic cross-section of an integratedcircuit package, in accordance with some embodiments.

FIG. 3 is a simplified diagrammatic cross-section of an integratedcircuit package, in accordance with some embodiments.

DETAILED DESCRIPTION

In the following description, like reference numbers are used toidentify like elements. Furthermore, the drawings are intended toillustrate major features of example embodiments in a diagrammaticmanner. The drawings are not intended to depict every feature of actualembodiments nor relative dimensions of the depicted elements and are notdrawn to scale.

Some embodiments described herein provide a semiconductor device thatincludes a top semiconductor die mounted on top of a bottomsemiconductor die. The bottom semiconductor die is flip-chip mountedonto a leadframe of a semiconductor device package such that a backsideportion of the bottom semiconductor die is oriented up, and a frontsideportion of the bottom semiconductor die is oriented down.

The backside portion of the bottom semiconductor die includes asemiconductor substrate, such as a handle wafer. The backside portion ofthe bottom semiconductor is electrically connected to a ground lead ofthe leadframe through an electrical contact of the frontside portion ofthe bottom semiconductor die. In some embodiments, this electricalconnection includes a conductive path through a buried insulator layerof the bottom semiconductor. The backside portion of the bottomsemiconductor, thus grounded, advantageously creates a ground planesituated below the top semiconductor die and above a frontside activelayer of the bottom semiconductor die. The presence of this ground planeadvantageously minimizes crosstalk between the top semiconductor die andthe bottom semiconductor die.

In some embodiments, the top semiconductor die is a controller die thatprovides control signals to circuit nodes (e.g., gate nodes oftransistors) formed in the bottom semiconductor die. In someembodiments, the bottom semiconductor die includes integrated lateraldiffusion field effect transistor (LDFET) circuits. Such LDFET circuitsinclude at least one substrate contact to the backside portion of thebottom semiconductor die to reduce the number of required frontsideelectrical connections. In this way, frontside space of the bottomsemiconductor die available to accommodate relatively large, highperformance, electrical connections (e.g., electrical connectors of aleadframe, such as metal strips capable of conducting high currents) isincreased. This, in turn, increases circuit design flexibility,performance, and manufacturability of integrated LDFET power devicecircuits. In some embodiments, an LDFET that has a substrate contact iselectrically isolated from other LDFETs in the same circuit to furtherimprove the performance of the circuit by preventing the formation of acommon node between the LDFETs that are connected to the substrate andthose that are not.

For illustrative purposes only, this disclosure describes specificexamples of single-semiconductor-die integrated LDFET circuits in thecontext of embodiments similar to an example high-power semiconductorswitch circuit 10 shown in FIG. 1. The same or similar teachings may beused to fabricate other single-semiconductor-die integrated LDFETcircuits that are suitable for both power and non-power applications.

The high-power semiconductor switch circuit 10 includes a high-sidefield effect transistor (FET) 12 and a low-side FET 14. The high-sidesource of the high-side FET 12 is coupled to the low-side drain of thelow-side FET 14 at a phase node 16 (V_(PHASE)). The high-side gate node18 and the low-side gate node 20 control the duty cycles of thehigh-side FET 12 and the low-side FET 14 to convert the input voltage(V_(IN)) at the voltage input node 23 at a high-side drain of thehigh-side FET 12 to an output voltage (V_(PHASE)) at the phase node 16.In general, the FETs 12, 14 may be fabricated using any of a widevariety of semiconductor material systems and technologies, includingsilicon, germanium, and compound semiconductor technologies.

FIG. 2A shows a top orthographic view of a simplified example of aportion of an integrated circuit (IC) package 200 that generallyincludes a leadframe 220, a top semiconductor die 222, and a bottomsemiconductor die 230, in accordance with some embodiments. The ICpackage 200 also generally includes bond-wires 292 a-d.

In general, the leadframe 220 includes electrical connectors 275 a-c andperimeter package leads 291 a-h. The number of electrical connectorsand/or perimeter package leads shown in FIG. 2A, or in any of thefigures herein, are shown as a simplified example. In some embodiments,more or fewer electrical connectors and/or perimeter package leads maybe used. Cross-section cutting lines 233 a-b are discussed later withreference to FIGS. 2B-C.

In the example shown, the bottom semiconductor die 230 embodies anintegrated point-of-load (POL) voltage converter, embodying an exampleimplementation of the high-power semiconductor switch circuit 10 shownin FIG. 1. However, the semiconductor device of the bottom semiconductordie 230 could be another semiconductor device as is known in the art. Inthe example shown, the bottom semiconductor die 230 generally includes abackside portion 231 a (e.g., a handle wafer) and a frontside (231 bshown in FIG. 2B) opposite the backside portion.

The top semiconductor die 222 generally includes top electrical contacts224 on a frontside 223 a, and a backside portion (e.g., a substrate suchas a handle wafer) (223 b shown in FIG. 2B) that is opposite thefrontside 223 a. In some embodiments, the top semiconductor die 222 isone of: (i) a controller die, (ii) a bulk semiconductor die, (iii) amicroprocessor, (iv) a microcontroller, (v) a digital signal processor,or (vi) another semiconductor as is known in the art. In the exampleembodiments described herein, the top semiconductor die 222 is acontroller die for a power switch circuit. In this case, the topsemiconductor die 222 is configured to synchronize the respective on/offstates of transistors of the POL voltage converter of the bottomsemiconductor die 230.

As shown, the backside of the top semiconductor die 222, opposite thefrontside 223 a, is physically mounted to the backside portion 231 a ofthe bottom semiconductor die 230. Respective electrical contacts (shownin FIG. 2B and FIG. 2C) of the frontside of the bottom semiconductor die230, opposite the backside portion 231 a, are electrically coupled toand physically mounted to the electrical connectors 275 a-c of theleadframe. The backside portion 231 a of the bottom semiconductor die230 advantageously creates a ground plane, upon which the topsemiconductor die 222 is mounted, thus reducing crosstalk between thetop semiconductor die 222 and the bottom semiconductor die 230.

One or more of the top electrical contacts 224 of the top semiconductordie 222 are electrically coupled to one or more of the perimeter packageleads 291 a-h, to the leadframe 220, and/or to the bottom semiconductordie 230 to receive or send signals, commands and/or feedback for controlof the electronic components, e.g., transistors, of the high-powersemiconductor switch circuit in the bottom semiconductor die 230. By wayof example, contacts of the top electrical contacts 224 of the topsemiconductor die 222 are electrically connected to the perimeterpackage leads 291 a-b by the bond-wires 292 a-b, and the electricalconnectors 275 b-c by the bond-wires 292 c-d.

FIG. 2B shows a diagrammatic cross-section view of a portion of thesimplified example integrated circuit (IC) package 200 including theleadframe 220 for a semiconductor device that was introduced in FIG. 2A,in accordance with some embodiments. The cross-section view of the ICpackage 200 is a cross-section taken through a portion of the IC package200 of FIG. 2A indicated by the cutting line 233 a of FIG. 2A.

The IC package 200 of FIG. 2B includes the elements introduced in FIG.2A, as well as mounting medium 215, a backside 223 b of the topsemiconductor die 222, and a frontside 231 b of the bottom semiconductordie 230.

As shown, the bond-wire 292 a electrically couples the perimeter packagelead 291 a to a frontside electrical contact of the top electricalcontacts 224 of the top semiconductor die 222. The backside 223 b of thetop semiconductor die 222 is physically mounted to the backside portion231 a of the bottom semiconductor die 230 by the mounting medium 215. Insome embodiments, the mounting medium 215 includes die attach adhesive,sintered silver, solder paste, thermally conductive adhesive, or anysubstance suitable for forming physical, thermal and/or electricalconnections. In some embodiments, the mounting medium 215 is anelectrically insulating material. In some embodiments, one or more otherintervening layers (e.g., a metal layer or an insulating layer) aresituated between the top semiconductor die 222 and the bottomsemiconductor die 230.

In some embodiments, frontside electrical contacts of the bottomsemiconductor die 230 include copper pillars or solder bumps. A firstfrontside electrical contact 280 of the bottom semiconductor die 230 iselectrically connected and physically mounted to the electricalconnector 275 a, a second frontside electrical contact 282 of the bottomsemiconductor die 230 is electrically connected and physically mountedto the electrical connector 275 b, and a third frontside electricalcontact 284 of the bottom semiconductor die 230 is electricallyconnected and physically mounted to the electrical connector 275 c(e.g., each by a die-attach adhesive or other suitable mountingtechnique).

In some embodiments, the first frontside electrical contact 280, thesecond frontside electrical contact 282, and the third frontsideelectrical contact 284 of the bottom semiconductor die 230 generallyrepresent top metal layers of multiple metal layers of the bottomsemiconductor die 230. The bottom semiconductor die 230 is inverted, orin a flip-chip configuration, so the “top”/“frontside” and the“bottom”/“backside” of the bottom semiconductor die 230 are shown on thebottom and top, respectively, in the drawing. Some metal layers,connections, bond-wires, or other features have been omitted forsimplicity. Intervening metal layers, conductive adhesive, copperpillars, solder bumps, or other metal bonding structures may be present.

In some embodiments, at least five electrical connections are madebetween the bottom semiconductor die 230, the leadframe 220, and/or thetop semiconductor die 222. With reference to FIG. 1, these fiveelectrical connections include an electrical connection to the voltageinput node 23 (V_(IN)), an electrical connection to the phase node 16(V_(PHASE)), an electrical connection of a low-side source of thelow-side FET 14 to the ground node, an electrical connection to thehigh-side gate node 18, and an electrical connection to the low-sidegate node 20.

In some embodiments, a backside electrical contact (not shown) on thebackside 223 b of the top semiconductor die 222 is electrically coupledto the backside portion 231 a of the bottom semiconductor die 230. Insome embodiments in which the mounting medium 215 is electricallyconductive, the mounting medium 215 is used for this electricalcoupling. In other embodiments in which the mounting medium 215 iselectrically insulating, the backside 223 b of the top semiconductor die222 is substantially electrically isolated from the backside portion 231a of the bottom semiconductor die 230 by the mounting medium 215. Ineither embodiment, the backside portion 231 a of the bottomsemiconductor die 230 advantageously provides a ground plane forminimizing crosstalk between the top semiconductor die 222, and thebottom semiconductor die 230.

FIG. 2C shows a diagrammatic cross-section view of a portion of thesimplified example integrated circuit (IC) package 200 including theleadframe 220 for a semiconductor device, in accordance with someembodiments. The cross-section view of the IC package 200 shown in FIG.2C is a cross-section taken through a portion of the IC package 200 ofFIG. 2A indicated by the cutting line 233 b of FIG. 2A.

The IC package 200 of FIG. 2C includes the elements introduced in FIGS.2A-B, as well as a fourth frontside electrical contact 286 of the bottomsemiconductor die 230. In some embodiments, the fourth frontsideelectrical contact 286 is electrically coupled to a gate contact of atransistor of the bottom semiconductor die 230. As shown, the fourthfrontside electrical contact 286 is electrically coupled to andphysically mounted to the perimeter package lead 291 b, which extendsunder the bottom semiconductor die 230. Although the fourth frontsideelectrical contact 286 is shown physically mounted to the perimeterpackage lead 291 b, in other embodiments, the fourth frontsideelectrical contact 286 is electrically coupled to the perimeter packagelead 291 b (e.g., by bond-wire), but is not physically mounted to theperimeter package lead 291 b.

In the embodiment shown, the bond-wire 292 b electrically couples theperimeter package lead 291 b to a frontside electrical contact of thetop electrical contacts 224 of the top semiconductor die 222 and therebyelectrically couples the fourth frontside electrical contact 286 to thefrontside electrical contact of the top semiconductor die 222.

In some embodiments, the fourth frontside electrical contact 286generally represents a top metal layer of multiple metal layers of thebottom semiconductor die 230. Some metal layers, connections,bond-wires, or other features have been omitted for simplicity.Intervening metal layers, conductive adhesive, copper pillars, solderbumps, or other metal bonding structures may be present.

FIG. 3 shows a simplified diagrammatic cross-sectional side view of aportion of an integrated circuit (IC) package 300 which includes theelements introduced in FIGS. 2A-C, including the leadframe 220, inaccordance with some embodiments. Portions of the leadframe 220, shownin earlier figures, have been omitted for simplicity. For example, theperimeter package leads 291 a-h of FIG. 2A, though part of the leadframe220, have been omitted from FIG. 3.

In the simplified example embodiment shown, the semiconductor die 230 isa power semiconductor die, embodying the high-power semiconductor switchcircuit 10 of FIG. 1. In this example, an LDFET 32 implements thehigh-side FET 12 of the switch circuit 10, and an LDFET 34 implementsthe low-side FET 14 of the switch circuit 10. In one exampleconfiguration, the first frontside electrical contact 280 of thehigh-side LDFET 32 corresponds to the voltage input node 23 of theswitch circuit 10, the second frontside electrical contact 282corresponds to the phase node 16 of the switch circuit 10, and asubstrate 45 (e.g., a handle wafer), electrically connected to the thirdfrontside electrical contact 284, corresponds to the ground node of theswitch circuit 10.

A drain region 52 of the LDFET 32 is electrically coupled to a draincontact 56 that is connected to the first frontside electrical contact280. A source region 46 of the LDFET 32 is electrically coupled to asource contact 54 that is electrically connected to the second frontsideelectrical contact 282. A drain region 52′ of the LDFET 34 iselectrically coupled to a drain contact 56′ that is electricallyconnected to the second frontside electrical contact 282. A sourceregion 46′ of the LDFET 34 is electrically coupled to the source contact54′ that is electrically connected to the third frontside electricalcontact 284. Thus, the substrate 45 of the bottom semiconductor die 230is electrically coupled to the third frontside electrical contact 284through the source contact 54′.

The third frontside electrical contact 284 is, in turn, electricallyconnected to the electrical connector 275 c, which is a ground voltagelead of the leadframe 220. By coupling the substrate 45 to ground, aground plane is formed between the top semiconductor die 222 and thebottom semiconductor die 230, since the substrate 45 is interposedbetween most, if not all, of the top semiconductor die 222 and the restof the layers of the bottom semiconductor die 230, thereby minimizingcrosstalk between the top semiconductor die 222 and the bottomsemiconductor die 230.

In some embodiments, frontside electrical contacts of the topsemiconductor die 222 (e.g., the top electrical contacts 224 shown inFIG. 2A) are electrically coupled to gate nodes of the LDFET 32 and/orthe LDFET 34 via wire bonds and/or through the leadframe 220. Thisconfiguration enables the top semiconductor die 222 to control theon/off state of the high-side LDFET 32 and of the low-side LDFET 34. Insome embodiments, the high-side LDFET 32 is a p-type transistor and thelow-side LDFET 34 is an n-type transistor. In other embodiments, both ofthe LDFETs 32, 34 are n-type transistors. In some embodiments, both ofthe LDFETs 32, 34 are switched simultaneously, either separately withtwo gate contacts, or together using a single gate contact (e.g., wherethe gate of the high-side LDFET 32 and the gate of the low-side LDFET 34are electrically coupled to a single gate contact).

In the example implementation of the high-power semiconductor switchcircuit 10 shown in FIG. 3, the drain contact 56 of the high-side LDFET32 is connected to the voltage input node 23 (with reference to FIG. 1),the source contact 54 of the high-side LDFET 32 and the drain contact56′ of the low-side LDFET 34 are both connected to the phase node 16(with reference to FIG. 1), and the source contact 54′ of the low-sideLDFET 34 is connected to the ground node (with reference to FIG. 1). Asmentioned above, other node connection arrangements are possible. Forexample, these other connection arrangements include any connectionarrangements between a first LDFET and a second LDFET that includes (i)a common node that electrically connects to a source of a first LDFETand a drain of a second LDFET, (ii) at least one of the drain of thefirst LDFET, the source of the second LDFET, and the common node iselectrically connected to the semiconductor substrate, and (iii) atleast two frontside electrical contacts that are connected to at leasttwo of the drain of the first LDFET, the source of the second LDFET, andthe common node that are not electrically connected to the semiconductorsubstrate.

The high-side and low-side LDFETs 32, 34 are implemented in a frontsideactive layer 42 of the bottom semiconductor die 230. The substrate 45 isin the backside portion 231 a of the bottom semiconductor die 230. Thefrontside active layer 42 can be any of a doped portion of the bulk of asemiconductor wafer, a localized well formed in a larger doped portionof a semiconductor wafer, an active layer of asemiconductor-on-insulator (SOI) wafer, and a localized well formed inan SOI wafer. In the illustrated example, the frontside active layer 42is a thin film formed over a buried oxide layer 44 on the substrate 45(e.g., an SOI substrate). In the illustrated example, a dielectricisolation barrier 47 extends between the high-side and low-side LDFETs32, 34 from the top of the frontside active layer 42 to the buried oxidelayer 44. In some examples, the dielectric isolation barrier 47 isformed using a shallow trench isolation (STI) process.

The high-side LDFET 32 includes the source region 46 formed in a dopedregion 48, a lightly doped drain (LDD) region 50 with a heavier dopedextension region 49 that are formed in a doped region 51, and the drainregion 52. The source region 46, the doped region 48, the LDD region 50,the extension region 49, and the drain region 52 can include dopedsemiconductor material formed by, for example, the implant of impuritiesinto the frontside active layer 42. The doped semiconductor material ofeach region 46, and 48-52 has a similar conductivity type (e.g., n-typeor p-type). Therefore, each region 46, and 48-52 can be formed by thesame dopant species, such as through the implant of one kind of dopantatom. The LDD region 50 has a lower dopant concentration than the drainregion 52 and may also have a lower dopant concentration than the sourceregion 46. The LDD region 50 provides the LDFET with superiorperformance as a power device in terms of its ability to hold off largevoltages and not degrade while sinking large currents. The presence ofthe LDD region 50 provides the LDFET with a characteristic of havingasymmetric source and drain regions. In some approaches, LDD region 50generally extends laterally at least twice as far from the drain region52 as the doped region 48 extends from the source region 46.

The high-side LDFET 32 also includes a body region 60 and a deep wellregion 62 that have a conductivity type that is opposite theconductivity type of the regions 46, and 48-52. The deep well region 62extends laterally underneath the source region 46 and the portion ofbody region 60 in which a channel forms. The deep well region 62enhances the ability of the high-side LDFET 32 to withstand largevoltages and serves to remove unwanted charge carriers from body region60 to prevent a parasitic bipolar junction transistor from activatingduring the on state of the high-side LDFET 32.

Above the frontside active layer 42, the high-side LDFET 32 includes agate structure that includes a gate shield 66 and a gate electrode 68.The gate electrode 68 is electrically insulated from the frontsideactive layer 42 and the gate shield 66 by dielectric materials 70, 72,respectively.

The drain region 52 can be a highly doped drain region and can form anelectrically conductive path between drain contact 56 and LDD region 50.Electrically insulating material 74 (e.g., an interlayer dielectric)electrically isolates and prevents unintended coupling of the electricalcomponents above the frontside active layer 42. In general, theelectrically insulating material 74 and the dielectric material 70, 72may be the same or similar materials. In addition, in certainapproaches, the combination of insulating material 74 and dielectricmaterial 70, 72 can be conceptualized as a single insulating layer inthe finished device regardless of when and how they are formed.

A conductive path is formed between source contact 54 and drain contact56 in response to the application of a voltage to gate electrode 68(e.g., by the top semiconductor die 222). The conductive path betweenthe source contact 54 and the drain contact 56 includes a channel thatis selectively formed in the body region 60 under the influence of theaforementioned voltage applied to the gate electrode 68. While thechannel is formed, the transistor is said to be on. While the channel isnot formed, and there is no conductive path between the source contact54 and the drain contact 56, the transistor is said to be off. There isno conductive path in this situation because the source region 46 andthe drain regions 50, 52 have an opposite conductivity type to bodyregion 60, such that diode junctions are formed at their interfaces.

The gate shield 66 is in ohmic contact with the source contact 54. Thegate shield 66 is another feature that makes the high-side FET 32 moreamenable to high-power applications. By biasing the gate shield 66 to agiven voltage, high-power signals on drain contact 56 are shielded fromhaving an appreciable effect on the gate region. Although the gateshield 66 is illustrated as being ohmically coupled to the sourcecontact 54, the gate shield 66 can also be independently biased. In someexamples, the gate shield 66 and the source contact 54 can be formed intwo different steps and can comprise two different kinds of material. Inthis case, however, such features are inconsequential to the operationof the device in most situations because the gate shield 66 and thesource contact 54 are one contiguous region of highly conductivematerial with an uninterrupted ohmic contact from above dielectricmaterial 74 all the way to the surface of the frontside active layer 42.As such, the combination of the gate shield 66 and the source contact 54can be conceptualized as a single source contact.

In general, the source contact 54 and the drain contact 56 enableelectrical connections to the high-side LDFET 32 from other circuitrythat may or may not be integrated with the LDFET on the same integratedcircuit. The source region 46 can be electrically coupled to the sourcecontact 54 via a silicide layer formed on the surface of the sourceregion 46. More generally, the source region 46 can be coupled to thesource contact 54 using any process that forms an ohmic ornon-rectifying contact between the two regions of the structure. Theconnection between the drain contact 56 and the drain region 52 cancomprise any of the variations described above with reference to thesource contact 54 and the source region 46. The source contact 54 andthe drain contact 56 can include a metal, metal alloy, metal silicide,or an electrically conductive semiconductor material such as dopedpolysilicon. Example metals, metal alloys, and metal silicides can eachcomprise copper, tungsten, molybdenum, and aluminum.

In the example shown in FIG. 3, some of the elements of the low-sideLDFET 34 of the frontside active layer 42 function in similar ways asthe corresponding elements of the high-side LDFET 32 of the frontsideactive layer 42. In this regard, the functionally similar elements ofthe low-side LDFET 34 are labeled with reference numbers of thecorresponding elements of the high-side LDFET 32 that are followed by anapostrophe. For example, a drain region of the low-side LDFET 34 thatcorresponds to the functionally similar drain region 52 of the high-sideLDFET 32 is labeled with reference number 52′. Thus, the low-side LDFET34 includes the following elements: the source region 46′, doped region48′, LDD region 50′ with a heavier doped extension region 49′ that areformed in a doped region 51′, the drain region 52′, the source contact54′, the drain contact 56′, body region 60′, deep well region 62′, gateshield 66′, gate electrode 68′, and dielectric material 70′, 72′.

In this example, the source contact 54′ of the low-side LDFET 34 notonly extends from above the frontside active layer 42, through thesource and doped regions 46′, 48′ to the deep well region 62′, but italso extends through the deep well region 62′ and the buried oxide layer44 and into the substrate 45. In this way, the source contact 54′ of thelow-side LDFET 34 provides a source-down electrical connection to thesubstrate 45 and thereby to the substrate 45, which corresponds to theground node for the high-power semiconductor switch circuit 10.

The second frontside electrical contact 282 (the phase node)electrically interconnects the source contact 54 of the high-side LDFET32 with the drain contact 56′ of the low-side LDFET 34 and, thereby,forms a common node for the source region 46 of the high-side LDFET 32and the drain region 52′ of the low-side LDFET 34. It is noted that theburied oxide layer 44 and the dielectric isolation barrier 47electrically isolate the high-side LDFET 32 from the substrate 45 toprevent the formation of a common node with the source contact 54′ ofthe low-side LDFET 34 during operation of the power switch circuit 10.

The semiconductor die 230 is mounted on and within portions of theleadframe 220 of the IC package 300. As shown, the first frontsideelectrical contact 280 of the bottom semiconductor die 230 iselectrically coupled to and physically mounted to the electricalconnector 275 a (an input voltage node lead of the leadframe 220), thesecond frontside electrical contact 282 is electrically coupled to andphysically mounted to the electrical connector 275 b (a phase node leadof the leadframe 220), and the third frontside electrical contact 284 iselectrically coupled to and physically mounted to the electricalconnector 275 c (the ground voltage lead of the leadframe 220).

Additional electrical connections between the top semiconductor die 222and the bottom semiconductor die 230, similar to those shown in FIG.2A-C, are made but are omitted in FIG. 3 for simplicity.

The backside 223 b of the top semiconductor die 222 is physicallymounted to the backside portion 231 a of the bottom semiconductor die230. The backside portion 231 a, which includes the substrate 45, iscoupled to electrical connector 275 c (the ground voltage lead) of theleadframe 220. This advantageously creates a ground plane between thetop semiconductor die 222 and the bottom semiconductor die 230 tominimize crosstalk between these dice.

Metal layers (e.g., the first frontside electrical contact 280) shown inFIG. 3 generally represent multiple metal layers that route connectionsas needed, including top metal layers for semiconductor die pads andadditional metal layers between the semiconductor die pads and theinsulating material (e.g., 74) of the frontside active layer 42. Somemetal layers, connections, bond-wires, or other features have beenomitted for simplicity. Intervening metal layers, conductive adhesive,copper pillars, solder bumps, or other metal bonding structures may bepresent.

The simplified diagrammatic cross-sectional side view shows only asingle transistor “finger” for simplicity. In some embodiments, multipletransistor fingers are connected in parallel to increase the powerhandling capability of the embodied circuit and to reduce totalresistance as required by the application of the embodied circuit.

Reference has been made in detail to embodiments of the disclosedinvention, one or more examples of which have been illustrated in theaccompanying figures. Each example has been provided by way ofexplanation of the present technology, not as a limitation of thepresent technology. In fact, while the specification has been describedin detail with respect to specific embodiments of the invention, it willbe appreciated that those skilled in the art, upon attaining anunderstanding of the foregoing, may readily conceive of alterations to,variations of, and equivalents to these embodiments. For instance,features illustrated or described as part of one embodiment may be usedwith another embodiment to yield a still further embodiment. Thus, it isintended that the present subject matter covers all such modificationsand variations within the scope of the appended claims and theirequivalents. These and other modifications and variations to the presentinvention may be practiced by those of ordinary skill in the art,without departing from the scope of the present invention, which is moreparticularly set forth in the appended claims. Furthermore, those ofordinary skill in the art will appreciate that the foregoing descriptionis by way of example only, and is not intended to limit the invention.

What is claimed is:
 1. A method comprising: providing a leadframe havingperimeter package leads and a ground voltage lead; forming a bottomsemiconductor die comprising: i) a first frontside active layer havingfirst frontside electrical contacts, ii) a first backside portion, andiii) a buried oxide layer, the buried oxide layer being situated betweenthe first frontside active layer and the first backside portion;flip-chip mounting the bottom semiconductor die to the leadframe, theflip-chip mounting comprising electrically connecting and physicallymounting the first frontside electrical contacts of the bottomsemiconductor die to the leadframe, a first electrical contact of thefirst frontside electrical contacts being electrically connected andphysically mounted to the ground voltage lead of the leadframe;providing a top semiconductor die, the top semiconductor die comprising:i) a second frontside having second frontside electrical contacts, andii) a second backside; and mounting the top semiconductor die to thebottom semiconductor die, the second backside of the top semiconductordie being attached to the first backside portion of the bottomsemiconductor die; wherein: the first frontside active layer of thebottom semiconductor die comprises a circuit electrically connected tothe first frontside electrical contacts by respective first frontsideelectrical connections and electrically connected to the first backsideportion by a first backside connection through the buried oxide layer;and the first backside portion of the bottom semiconductor die iselectrically connected to the ground voltage lead of the leadframethrough the first backside connection and the first electrical contactof the first frontside electrical contacts to minimize crosstalk.
 2. Themethod of claim 1, wherein: an electrical contact of the secondfrontside electrical contacts of the top semiconductor die iselectrically coupled to a first set of the perimeter package leads. 3.The method of claim 1, wherein: the bottom semiconductor die comprisestwo or more transistors; at least one of the two or more transistors iselectrically connected to the first frontside electrical contacts of thebottom semiconductor die; and at least one of the two or moretransistors is electrically connected to the first backside portion ofthe bottom semiconductor die.
 4. The method of claim 3, wherein: the twoor more transistors comprise a high-side transistor and a low-sidetransistor; the high-side transistor comprises a high-side source, ahigh-side drain, and a high-side gate; the low-side transistor comprisesa low-side source, a low-side drain, and a low-side gate; and the firstbackside portion of the bottom semiconductor die is electricallyconnected to the low-side source through the buried oxide layer.
 5. Themethod of claim 4, wherein: a top electrical contact of the secondfrontside electrical contacts of the top semiconductor die iselectrically connected to one or both of the high-side gate or thelow-side gate.
 6. The method of claim 5, wherein: the top electricalcontact is electrically connected to the one or both of the high-sidegate or the low-side gate through a perimeter package lead of theperimeter package leads of the leadframe.
 7. The method of claim 4,wherein: the first electrical contact of the first frontside electricalcontacts of the bottom semiconductor die is electrically connected tothe low-side source.
 8. The method of claim 7, wherein: a secondelectrical contact of the first frontside electrical contacts of thebottom semiconductor die electrically couples the low-side drain to thehigh-side source; and the second electrical contact is electricallycoupled and physically mounted to a phase node lead of the leadframe. 9.The method of claim 8, wherein: a third electrical contact of the firstfrontside electrical contacts of the bottom semiconductor die iselectrically coupled to the high-side drain; and the third electricalcontact is electrically coupled and physically mounted to an inputvoltage node lead of the leadframe.